A319 air carrier Captain reported encountering severe turbulence during descent with one flight attendant requiring medical attention.

Narrative

Descending via ZZZZZ passing through 11500 ft. experienced a severe turbulence event. Through decent from cruise very light turbulence no clouds in sky. ATC did not have any prior reported turbulence in vicinity. We doubled chimed Flight Attendant (FA) early to sit down at about 12000 ft due to gusty winds reported in ZZZ. 30 seconds to a minute later we were between ZZZZZ1 and ZZZZZ2 we encountered rapid and sudden pitch up and aircraft rolled to the right 25 degrees. I expected autopilot to turn off but it remained on. I called on FA to find out if they or any passengers (pax) were injured. I was told that Person A hit ceiling and wanted medical attention when we arrived at the gate and FA would get back to me if any pax required medical attention. While I talked to FA Fist Officer (FO) reported severe turbulence event we encountered to Approach control. We encountered no other significant turbulence prior to landing. At gate with brakes set made PA announcement when I turned off seat belt sign that medical personnel were meeting us at gate for injured FA and any pax that also needed or wanted medical treatment to inform flight crew for medical assistance. No pax came forward or notified FA for medical assistance; all pax were seated with seat belts fastened from top of descent ATC to notify flight crew if turb is expected in descent.